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Bonding to Existing Concrete

posted by Bob Monday, November 22, 2010 at 12:00 PM

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jack hammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also keep your thumb out of the way. If the spots are too large or too deep for this to be practical you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it just ain’t so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder/Fortifier. When you use a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day the existing concrete surface will be hot enough to suck the water our of the repair material. In addition some concretes are quite porous and will also rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result.

There are some substances that concrete simply will not bond to. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.



Steve P, yes, by adding less water to the mix you will be able to apply the Sakrete Flo-Coat in a trowel grade consistency. A slurry coat which will act as a primer prior to the application of the Flo Coat is recommended for best adhesion. The Slurry mixture is a 1:1 powder and water mixed together to a thin paintable consistency. Let the slurry coat dry before application of the trowelable material. Finish can be done using an inexpensive paint brush to remove any trowel marks and to ensure the product is not over worked.
- Chris Technical Services
Monday, September 8, 2014 at 4:45 PM

Leo A, TCNA (Tile Council of North America) which follows ANSI (American National Standard Specifications) specifications for ceramic tile installation specifies a slope of a ¼” per foot to the drain for proper drainage which you exceed based on your slope given. The product we would recommend for your project is the Sakrete Sand Mix. For a “Mud Bed” (shower tile base) the recommended mix consistency should be what is called a dry pack. It should have the feel of compacted wet sand and when you make a ball out of the mix it should hold together. After applying it and screeding it you take a float and pound down to pack it tight. We suggest that you follow the installation guidelines in the TCNA handbook for all shower and tile installations. If you should have any additional questions please do not hesitate to contact us here at Sakrete.
- Chris Technical Services
Monday, September 8, 2014 at 4:43 PM

Jim, first we want to thank you for choosing Sakrete for your repair. To assure that you have a successful repair and durable surface the first thing that needs to be addressed and the preparation of your sidewalk. You can prep the sidewalk by removing any dirt, dust, loose material, paint, stain, sealers, etc. or any foreign materials that will inhibit the repair material to bond to the existing concrete. The product that would work best for your project is the Sakrete Sand Mix. It is designed for applications from ½” up to 2” in thickness. Along with the Sand Mix we would recommend that you modify the mix using Sakrete Bonder and Fortifier as an admixture (50/50 ratio with water to suggested amount) which will give you superior adhesion and added strength and durability. For the areas that require ½” down to a feathered edge we would recommend using Sakrete Top ‘N Bond. Top ‘N Bond is a highly polymer modified material that has pronounced adhesion properties and will allow you to blend easily. Do not add a bonding agent to the Top ‘N Bond since it is already polymer modified. Be sure to honor the expansion joints that separate the “3 concrete squares” If you do bridge the expansion joints cracking of the repair material will occur.
- Chris Technical Services
Thursday, September 4, 2014 at 1:22 PM

My front patio needs to be resurfaced to cover pitting from either lack of air entrainment or poor finishing. Concrete is structurally fine. What product would adhere to the 8-10" vertical front surface of the porch? Flo Coat appears to be too thin for this, or can it be mixed with less water to allow it to adhere vertically?
- Steve P
Wednesday, September 3, 2014 at 10:12 PM

The area where I'm going to install my stand-up shower is not level. For 42" length the slope is 1-1/8". What concrete I could apply in this situation?
- Leo A.
Wednesday, September 3, 2014 at 7:01 PM

Phil, you do not want to apply a rigid material to 2 dissimilar substrates. You can however fill the gap using Sakrete Backer Rod leaving up to ½” depth space from the top and seal with Sakrete Polyurethane Non Sag Sealant. This will give you a flexible joint between a horizontal and vertical substrate.
- Chris Technical Services
Wednesday, September 3, 2014 at 1:22 PM

Could you advise me on a repair on a front city sidewalk? The (3) concrete squares have settled unevenly, and the city has a manhole access next to the center square. The patch will need to be approx. 1-1/2" thick at the deepest point and taper out to nothing at the thinnest point. Will the patch adhere to the old concrete without flaking off in the thin areas? Thanks for any advice or comments!
- Jim S.
Monday, September 1, 2014 at 2:57 PM

Can I use Sakrete top bond or flo-coat to fill the 1x4 void where the wood for the footing form tie has rotted away and is allowing water into the crawlspace?
- Phil
Sunday, August 31, 2014 at 11:21 AM

LG, it sounds like you are setting a post on top of a footing that has already been poured. The rebar help join the existing concrete to the new and give added strength. We suggest using Sakrete Fast Setting Concrete Mix for this type of application. After your rebar is in place and the post is level fill the hole 1/3 full of water and pour in the Fast Setting Concrete directly from the bag filling the hole to the top. If the surface is dry add more water. Excess surface water is not a problem. Slope top of mix to allow water to drain away from the post.
- Chris Technical Services
Friday, August 29, 2014 at 9:16 AM

Dave, Unfortunately this cannot be accomplished by using the Sakrete Flo-Coat. We would not have any recommended materials through Sakrete for this type of application.
- Chris Technical Services
Tuesday, August 26, 2014 at 4:30 PM



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